#include <iostream>
#include <vector>
#include <queue>
#include <stack>
#include <algorithm>
using namespace std;

struct TreeNode
{
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode() : val(0), left(nullptr), right(nullptr) {}
    TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
    TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
};

class Solution {
public:
    int maxDepth(TreeNode* root) {
        queue<TreeNode *> que;
        int depth = 0;
        if (root) que.push(root);
        while(!que.empty()){
            int size = que.size();
            while(size--){
                TreeNode * node = que.front();
                que.pop();

                if (node->left) que.push(node->left);
                if (node->right) que.push(node->right);
            }
            depth++;
        }
        return depth;
    }
};